Hydrogeochemical and microbiological effects on fractures in the Excavation Damaged Zone (EDZ)

Due to the disturbances associated with the excavation, construction and closure of the repository for storage of spent nuclear fuel, the saturation state of the groundwaters at repository depth with respect to several mineral phases may change and mineral precipitation/dissolution reactions may take place. In addition, changing groundwater conditions may facilitate microbial growth on fracture walls. These processes are of importance since they may influence the stability and safety of the Excavation Damaged Zone (EDZ) because precipitation and microbial growth may seal the hydraulically conductive fractures caused by the repository construction. Different processes expected to occur in the EDZ during the open repository conditions and after repository closure have been evaluated based on data from Forsmark, Laxemar and Aespoe. Geochemical modelling by using PHREEQC was applied to simulate the following cases: - increase of temperature to 50 deg C and 100 deg C to simulate the thermal effects from spent nuclear fuel; - open repository conditions simulating atmospheric conditions (equilibrium with atmospheric partial pressures of CO2(g) and O2(g)); - mixing with deep saline water simulating up-coning; - mixing with shallow infiltration waters simulating down-coning; - mixing with different proportions of cement dissolution porewater. The effect of variable temperatures (up to 100 deg C) on most of the above modelled processes has also been assessed. A preliminary estimation of the effect of mineral precipitation on the hydraulic conductivity of the EDZ has been carried out. For most of the modelling cases, the estimated decrease of the hydraulic conductivity in ten years is smaller than 2%. Microbial evaluation was used to identify the potential for microbial calcite and iron hydroxide formation during various repository conditions. The most important groundwater parameters for microorganisms, are pH and carbonate, ferrous iron, methane and the dissolved organic carbon (DOC) concentrations. The following cases where evaluated: - The potential for microbial iron hydroxide production will be large in all groundwater with ferrous iron. The production will be significant as soon as the oxygen concentration rises above 0.3 mg/L. This will trigger an increase in the Eh towards the range where iron oxidation is favourable (> 100 mV). The potential for microbial calcite formation production will be large in all groundwater with high concentrations of DOC and methane, irrespective of oxygen, Eh or pH. - Deep groundwater generally has less ferrous iron and DOC than intermediate and shallow groundwaters. It can, therefore, be assumed that the rate of microbial iron oxide and calcite formation will decrease, relative to the unchanged situation, except for cases where deep groundwater contains a high concentration of methane that mixes with a sulphate or oxygen rich groundwater. - Shallow groundwater generally has more ferrous iron and DOC (and less methane) than intermediate (unchanged) groundwater. It can, therefore, be assumed that the rate of microbial iron oxide and calcite formation will increase in relation to the unchanged situation. - The contamination explosives with nitrogen compounds will not influence the concentrations of ferrous iron, DOC or methane and the situation will, therefore, not change in relation to the unchanged situation. - The increase in pH from cement grouting will not have a large effect on the microorganisms. However, observations made in tunnels elsewhere where low pH cement has been injected reveal vivid microbial growth and formation of up to 5 cm thick slime layers on the tunnel walls, the slime clogged water conducting fractures. - There is no information available about the potential for microbial growth and activity at 50 deg C and 100 deg C. As a rule of thumb, biological processes double with every 10 deg C increase in temperature. Increasing temperature will then most probably speed up the bio-precipitating processes. It is important to note that the calculations included in this report correspond to a preliminary assessment of the groundwater sealing potential. More sophisticated calculations are required in order to increase the accuracy of the modelling and especially to be able to quantify the effects from microbial sealing. Some of these improvements include the incorporation of mineral dissolution reactions able to increase the concentrations of dissolved elements, the evaluation of kinetic vs. equilibrium approaches to the dissolution/precipitation reactions, the refinement of the thermodynamic description of the cement and grout constitutive phases and the inclusion of more realistic recipes for the low-pH cement, and probably, the implementation of reactive transport models of the EDZ and its surroundings. Testing and developing a code for microbial modelling and moreover include more site-specific physical and hydraulic parameters from the repository geological system, such as porosities, permeabilities or fracture densities and width, are needed if more accurate results about fracture sealing are required.